没有特殊关键字的简单LOOP表单：
(loop forms...)
为了打破循环，我们可以使用(return <return value>)`
一些例子：
(loop (format t "Hello~%")) ; prints "Hello" forever (loop (print (eval (read)))) ; your very own REPL (loop (let ((r (read))) (typecase r (number (return (print (* r r)))) (otherwise (format t "Not a number!~%")))))